Huestis: Observing the Last Quarter Moon

There is a lunar phase that is not as regularly observed as other phases. It is the last (or third) quarter Moon. And the reason for its lack of scrutiny by casual stargazers is because it generally rises around midnight. Although space does not permit me to review the reason for the cycle of lunar phases, you can refresh your understanding by visiting here.

March’s last quarter Moon will occur on the 20th, so this month’s column will highlight a few of the features that can be observed during this often overlooked phase. A small inexpensive telescope will be required to observe these formations adequately, though binoculars or even a birder’s spotting scope will provide glimpses of a few of them.

Even around the midnight hour you’ll need to wait for the moon to ascend higher into the sky and above the tree line. But if you don’t wish to forgo your beauty sleep, the next best time to observe this phase is a couple of hours before morning twilight. At sunrise the last quarter Moon will be due south of your location and at its highest point off the horizon. You can easily observe the moon in broad daylight, but the contrast is low, causing fewer details to be seen.

The accompanying Moon map will help you locate the lunar features I will explore. The Moon’s north pole is at the top of the map, while its western limb is to the left. Keep in mind that binoculars will provide you the same view that the map shows. Various telescope designs reverse the image right to left and up and down. You’ll need to orient your telescopic view to the map using a prominent surface feature as a guide.

There are many formations that can be observed during this lunar phase. And even if you have observed some of them during a different phase of illumination, the ever changing sunlight angle can reveal subtle details not seen in those other phases.

Also, most native Rhode Islanders know that our state is often used as a unit of measure. Refer to the inset of the Rhode Island state map to scale in the un-illuminated half of the moon map. East to west “Little Rhody” is approximately 37 miles across, and the north to south dimension measures 48 miles. Keep these figures in mind when comparing to crater sizes.

On the edge of Mare Imbrium (translation means Sea of Rains – yes, early astronomers once thought the Moon possessed seas and oceans) is the crater Archimedes, an almost perfect circle about 52 miles across. Under low magnification its floor is almost featureless. A short distance to the southeast and seeming to extend from the terminator (during Last Quarter the terminator defines the sunset point) is the Apennine Mountains. This range contains some of the tallest mountains on the lunar surface. Carefully scan up and down this region. Some of the mountains’ bases may already be in shadow while their peaks can still be catching glimpses of sunlight.

As we continue our journey south and to the west we encounter the absolutely beautiful crater Copernicus. While this crater is not the largest (only 53 miles across), the detail one can observe is remarkable. Its walls show very fine detail and the crater floor has an incredible central peak. In fact, during one perfect evening many moons ago while using the 8¼-inch Clark refractor at Seagrave Observatory, I was able to look deep into this crater and see where a huge boulder had tumbled down one of its steep walls. Outstanding!

Next please locate and examine the crater chain that comprises Ptolemaeus, Alphonsus and Arzachel. The detail that can be seen here is exquisite. Ptolemaeus is an old crater about 95 miles in diameter. Another impact, Herschel, blasted a 25-mile-in-diameter hole into its northern rim, and Ptolemaeus also shows smaller impacts on its floor. Alphonsus, 74 miles across, has a well- preserved central peak, where Ptolemaeus does not. Arzachel is roughly 60 miles in diameter and is very well preserved, showing great detail in its walls and central peak.

One of the most fascinating features on the lunar surface is the Straight Wall. This feature lies to the southwest of Arzachel and sits in Mare Nubium (Sea of Clouds). This formation is very impressive. It is a fault or escarpment approximately 68 miles long, 1.5 miles wide, and no more than 1,000 feet above the floor of the Mare. While it may look very steep, its slope is no more than 7 degrees. The Straight Wall’s appearance changes dramatically with the sun angle, so try to observe it during other lunar phases as well.

And finally I can’t end this lunar tour without noting crater Clavius. Sci-fi fans will recall that the monolith in the movie 2001: A Space Odyssey was uncovered in this crater. Clavius is huge, measuring 140 miles across. Though several smaller impacts have marred its floor, the inner crater walls are high and well defined. Several impacts have also occurred along the rim.

I hope this brief tour of our closest neighbor in space will encourage you to spend a few hours examining the lunar surface with whatever optical instrument you can use. The more magnification one is able to apply, the more detail one will be able to discern. Binoculars and telescopes should be outside collecting moonlight, not inside collecting dust in a closet or basement.

In conclusion, don’t forget we set our clocks ahead one hour (spring ahead) to EDT (Eastern Daylight Time) on Sunday morning, March 12, at 2:00am. On this date most of the United States shifts to Daylight Saving Time. And finally, on the same day as the last quarter moon (March 20), the Vernal Equinox (spring) begins at 6:29am EDT. The Sun appears to be moving northward in our sky as a result of the Earth’s axial tilt as we revolve around the Sun on our axis.

The only observatory you may often find open after midnight to observe the last quarter Moon is Frosty Drew Observatory in Charlestown, open every clear Friday night year-round. However, don’t forget that the other observatories are open at more convenient times to observe the heavens. Seagrave Memorial Observatory in North Scituate is open to the public every clear Saturday night. Ladd Observatory in Providence is open every clear Tuesday night. The Margaret M. Jacoby Observatory at the CCRI Knight Campus in Warwick is open every clear Wednesday night.

Great American Total Solar Eclipse on August 21, 2017. Countdown: 172 days as of March 1, 2017.